Detection of a specific bacterial antigen in urine of rats with Bacteroides fragilis infection.

Human intra-abdominal infections frequently yield Bacteroides fragilis and require specific antimicrobial and surgical therapy. Noninvasive immunologic assessment of this organism might allow more optimum therapy. Therefore we raised antisera in goats to Bacteroides fragilis ATCC 23745 and allowed it to react with a solid-phase capsular polysaccharide-protein antigen extracted from the same organism. Preliminary work disclosed that 10 ng/ml antigen could be detected in competition assays in both saline and dialyzed rat urine. Results were manifest by diminution of bound antiglobulin alkaline phosphatase conjugate in an antigen-mediated antibody-inhibition enzyme-linked immunosorbent assay. Rats were then infected intra-abdominally with (1) B. fragilis ATCC 23745; (2) one of eight recent clinical isolates of B. fragilis; or (3) one of nine isolates representative of Enterobacteriaceae. Seventy-two rat urine samples obtained prior to infection disclosed essentially no assay inhibition: 98.3% +/- 10.3 (1 S.D.). Mean values of reagent antibody activity after incubation with urine aliquots from 24 hr samples collected between 24 and 72 hr were (1) strain 23745 (n = 35) 70.9% +/- 2.6 (S.E.); (2) eight isolates of B. fragilis (n = 49) 86.8% +/- 1.9; (3) nine isolates of Enterobacteriaceae (n = 47) 100.9% +/- 1.0; and (4) shams (n = 29) 95.5% +/- 1.55. Ascribing values less than or equal to 77.7% (2 S.D.) as positive, seven of the eight clinical B. fragilis isolates causing infection were detected in at least one 24 hr urine sample (sensitivity = 87% by organism); 12 of 17 infected rats were correctly identified as positive by at least one urine (sensitivity = 70.6% by rat). Specificity, as assessed in the Enterobacteriaceae group, was 89% (by organism) and 94.5% (by rat). Collectively, these results suggest the presence of a potentially specific, soluble antigen excreted in the urine of rats with B. fragilis infection.